Various mobile communication devices and mobile electronic devices such as laptop computers have emerged in recent years, and this has lead to a demand for higher capacity batteries as their driving power sources. Lithium secondary batteries, which perform charge-discharge operations by transferring lithium ions between the positive and negative electrodes, have been widely used as the driving power source for the mobile communication devices and the like since the lithium secondary batteries have higher energy density and higher capacity than other types of secondary batteries such as nickel-cadmium batteries. Nevertheless, as the mobile communication devices and other electronic devices have increasingly become smaller and lighter, there has been a demand for further improvements in the energy density and cycle performance of the lithium secondary batteries.
Currently, in common lithium secondary batteries, carbon materials such as graphite are generally used as their negative electrode materials (negative electrode active materials). When using a negative electrode material composed of graphite, lithium occlusion is only possible up to the composition LiC6, and the upper limit of battery capacity is limited to the theoretical capacity 372 mAb/g. This has been an obstacle to achieving a higher battery capacity.
In view of this problem, a lithium secondary battery employing aluminum, silicon, or tin that alloys with lithium as a negative electrode active material with a high energy density per mass and per volume has been reported (see Non-patent Reference 1 indicated below). Among the just-mentioned materials, silicon particularly shows a high theoretical capacity and is therefore promising as a negative electrode active material for the batteries that can achieve a high capacity. Various lithium secondary batteries using silicon as the negative electrode active material have been proposed (see Patent Reference 1 indicated below).
When silicon is used as the negative electrode active material, however, the negative electrode active material undergoes expansion and shrinkage, and consequently, each time the charge and discharge are performed, newly exposed surfaces form in the surface of the negative electrode active material, causing the negative electrode active material to react with the non-aqueous electrolyte. This leads to the problem of deterioration in battery cycle performance. Moreover, the expansion of the negative electrode active material causes an increase in the battery thickness.
To resolve the problems, there has been a proposal to control the reactivity in the newly formed surfaces in the negative electrode active material by improving the non-aqueous electrolyte (see Patent Reference 2 indicated below). Techniques for improving cycle performance by using a negative electrode active material with a small crystallite size have also been proposed (see Patent References 3 and 4 indicated below).    [Patent Reference 1] Japanese Published Unexamined Patent Application No. 10-255768    [Patent Reference 2] WO2004/109839    [Patent Reference 3] Japanese Published Unexamined Patent Application No. 2004-319390    [Patent Reference 4] Japanese Published Unexamined Patent Application No. 2004-311429    [Non-patent Reference 1] Solid State Ionics, 113-115, p. 57